Epitaxial Si, Ge, Si.sub.x Ge.sub.1-x and Si.sub.x GeYC.sub.1-x-y thin films are used extensively in the production of Si-based microelectronic devices. Si is typically deposited on blanket Si substrates by the high temperature pyrolysis of one of the chlorosilane precursors, such as dichlorosilane. The use of silane SiH.sub.4 as a precursor reduces the required substrate temperature. The use of a plasma assisted deposition process has the ability to reduce the substrate temperature for this growth process even further. The associated gaseous discharge assists in the creation of the precursors to film growth, such as SiH, SiH.sub.2 and SiH.sub.3, and also assists in the removal of impediments to film growth, e.g., hydrogen on the substrate surface. The movement of chemical species on the growth surface is enhanced by the energetic particle flux to the surface, which in general assists in the overall growth process. It is known that growth of epitaxial layers at lower substrate temperatures reduces unwanted autodoping, dopant diffusion and the creation of crystal defects. The ability to epitaxially deposit Si-based and Ge-based films at reduced substrate temperatures would improve the quality of semiconductor devices, would simplify the process used to fabricate such devices, and would make possible the fabrication of new device structures.
Currently there is considerable interest in the growth of Si.sub.x Ge.sub.1-x and Si.sub.x Ge.sub.y C.sub.1-x-y alloys for producing heterojunction devices. Successful epitaxial growth of these heteroepitaxial alloys on Si substrates requires that low substrate temperatures (i.e., below 650.degree. C.) be used to avoid relaxation of the pseudomorphic crystal structure. Unfortunately, the growth rate at such low substrate temperatures is presently quite small, on the order of 50 .ANG./minute.
Chemical vapor deposition (CVD) is a process in which chemical species in the vapor phase react on a heated substrate surface to produce a solid material film. Plasma assisted chemical vapor deposition (PECVD) is a specific variety of a CVD process in which energetic electrons in a gaseous discharge are used to assist in the production of reactive chemical species, principally in the gas. These species then participate in the deposition process. Further, energetic ion bombardment of the substrate surface may be included to aid in the growth process.
The ability to deposit epitaxial semiconductor films, doped or otherwise, is a critical technology required in fabricating integrated circuits. It has long been recognized that the ability to deposit semiconductor epitaxial films at reduced substrate temperatures will be required for the fabrication of subsequent generations of integrated circuit devices. In addition to the advantages stated above, low substrate temperature epitaxial growth of semiconductor films would permit the deposition process to be performed outside of thermal equilibrium. These advantages would result in a reduction in linewidth and junction depth of present device designs, and would permit the fabrication of new device structures currently limited by high processing temperatures.
The ability to grow epitaxial semiconductor films at reduced substrate temperatures using plasma assisted processes has been proven in numerous investigations. The exact role of the plasma in the deposition process has been attributed to a number of factors, as described in the article by W. J. Varhue, P. S. Andry, J. L. Rogers, E Adams, R. Kontra and M. Lavoie, entitled "Low temperature growth of Si by PECVD," Solid State Technology, 163 June (1996). These factors include: production of reactive species which are the precursor to film growth, the removal of adsorbed hydrogen from the growth surface which prevents the adsorption of growth species on the surface, and the increase in adatom surface mobility to lower the required deposition temperature.
Despite the recognized ability of plasma assisted processes to lower the required substrate temperature for epitaxial growth, such processes have failed to be accepted by semiconductor device manufacturers due to the low deposition rate (about 50 .ANG./min) and low resulting thin film quality.
To date, there have been attempts to grow Si and Si.sub.x Ge.sub.1-x compounds at low substrate temperatures at higher deposition rates. For example, U.S. Pat. No. 4,579,609 to Reif et al. ("the '609 patent") discloses a process and apparatus for epitaxially growing Si films by a PECVD process. The specification states, in column 7, lines 43-48, that "specular epitaxial films have been deposited at temperatures as low as 650 .degree. C. using low pressure CVD both with and without plasma enhancement. This is the lowest silicon epitaxial deposition temperature and the lowest pre-epitaxial cleaning temperature believed reported for thermally driven CVD." However, the inventors, J. H. Comfort and R. Reif, in a subsequent paper entitled "Chemical Vapor Deposition of Epitaxial Silicon from Silane at Low Temperatures", Journal of the Electrochemical Society 136 (8) 2430 (1989) state the same process would not work at temperatures below 700 .degree. C.
Further, the '609 patent does not emphasize the role of energetic ion bombardment of the substrate surface during the growth process, which, as discussed below, is a key aspect in achieving low temperature growth of Si-based films.